avoid 416 response teardown binding to null pointer

avoid 416 response teardown binding to null pointer

methods/http.cc:640:13: runtime error: reference binding to null pointer
of type 'struct FileFd'

This reference is never used in the cases it has a nullptr, so the
practical difference is non-existent, but its a bug still.

Reported-By: gcc -fsanitize=undefined
(cherry picked from commit 4460551841d909d3ee9c1de00156ed3cdf8b1665)